Method of forming metal contacts in the barrier layer of a group iii-n hemt

ABSTRACT

Metal contact openings are etched in the barrier layer of a group III-N HEMT with a first gas combination that etches down into the barrier layer, and a second gas combination that etches further down into the barrier layer to a depth that lies above the top surface of a channel layer that touches and lies below the barrier layer.

RELATED APPLICATIONS

This application is a continuation of U.S. Nonprovisional patentapplication Ser. No. 13/856,016, filed Apr. 03, 2013, which is relatedto U.S. Nonprovisional patent application Ser. No. 13/856,043 (TI-71732)for “Method of Forming Metal Contacts with Low Contact Resistances in aGroup III-N HEMT” by Yoshikazu Kondo et al., the contents of both ofwhich are herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

The present invention relates to a method of forming group III-N HEMTsand, more particularly, to a method of forming metal contacts in thebarrier layer of a group III-N HEMT.

2. Description of the Related Art.

Group III-N high electron mobility transistors (HEMTs) have shownpotential superiority for power electronics due to their wider bandgapand high electron saturation velocity. These material propertiestranslate into high breakdown voltage, low on-resistance, and fastswitching. Group III-N HEMTs can also operate at higher temperaturesthan silicon-based transistors. These properties make group III-N HEMTswell suited for high-efficiency power regulation applications, such aslighting and vehicular control.

A conventional group III-N HEMT includes a substrate, and a layeredstructure that is formed on the top surface of the substrate. Thelayered structure, in turn, includes a buffer layer that lies on thesubstrate, a channel layer that lies on the buffer layer, and a barrierlayer that lies on the channel layer. Further, the layered structure canoptionally include a cap layer that lies on the barrier layer.

The buffer layer provides a transition layer between the substrate andthe channel layer in order to address the difference in lattice constantand to provide a dislocation-minimized growing surface. The channellayer and the barrier layer have different polarization properties andband gaps that induce the formation of a two-dimensional electron gas(2DEG) that lies at the top of the channel layer. The 2DEG, which has ahigh concentration of electrons, is similar to the channel in aconventional field effect transistor (FET). The cap layer enhances thereliability of the group III-N HEMT.

A conventional group III-N HEMT also includes a metal gate that isformed on the top surface of the layered structure. The metal gate makesa Schottky contact to the barrier layer (or the cap layer if present).Alternately, the metal gate can be isolated from the barrier layer (orthe cap layer if present) by an insulating layer.

In addition, a conventional group III-N HEMT includes a source metalcontact and a drain metal contact that lies spaced apart from the sourcemetal contact. The source and drain metal contacts, which lie in metalcontact openings that extend into the layered structure, make ohmiccontacts with the barrier layer.

Native group III-N substrates are not easily available. As a result,group III-N HEMTs commonly use a single-crystal silicon substrate.(Silicon carbide is another common substrate material for group III-NHEMTs.) The layered structure is conventionally grown on the substrateusing epitaxial deposition techniques such as metal organic chemicalvapor deposition (MOCVD) and molecular beam epitaxy (MBE).

Each of the layers in the layered structure is typically implementedwith one or more sequential group-III nitride layers, with the group-IIIincluding one or more of In, Ga, and Al. For example, the buffer layercan be implemented with sequential layers of AlN (a thermally-stablematerial), AlGaN, and GaN. In addition, the channel layer is commonlyformed from GaN, while the barrier layer is commonly formed from AlGaN.Further, the cap layer can be formed from GaN.

The source and drain metal contacts are conventionally formed by forminga passivation layer, such as a silicon nitride layer, on the top surfaceof the layered structure (on the top surface of the cap layer ifpresent, or the top surface of the barrier layer when the cap layer isnot present). Following this, a patterned photoresist layer is formed onpassivation layer.

After the patterned photoresist layer has been formed, the exposedregions of the passivation layer, the underlying portions of the caplayer (if present), and the underlying portions of the barrier layer aredry etched for a predetermined period of time using a gas combinationthat includes CHF₃, CF₄, Ar, and O₂.

The dry etch forms source and drain metal contact openings that extendthrough the passivation layer, through the cap layer (if present), andinto the barrier layer. It is very difficult to control the depths ofthe metal contact openings because the etch is very short, typically afew seconds. As a result, the bottom surface of the metal contactopenings frequently extends through the barrier layer and into thechannel layer.

After this, a metal layer is deposited to lie over the passivation layerand fill up the metal contact openings. The metal layer is thenplanarized to expose the top surface of the passivation layer and formsource and drain metal contacts in the source and drain metal contactopenings, respectively.

SUMMARY OF THE INVENTION

The present invention provides a method of forming metal contacts inmetal contact openings that expose the barrier layer without exposingthe channel layer. The method includes etching a layered structure witha first gas combination to form a number of metal contact openings. Thelayered structure includes a buffer layer that touches and lies over asubstrate, a channel layer that touches and lies over the buffer layer,and a barrier layer that touches and lies over the channel layer. Eachof the metal contact openings has a first bottom surface that lies aboveand spaced apart from a top surface of the channel layer. The methodalso includes etching the layered structure with a second gascombination to deepen the first bottom surface of each metal contactopening a distance to a second bottom surface that lies below the firstbottom surface. The second bottom surface lies above and spaced apartfrom the top surface of the channel layer.

The present invention also provides an alternate method of forming metalcontacts in metal contact openings that expose the barrier layer withoutexposing the channel layer. The method includes etching a barrier layerwith a gas combination that includes boron trichloride (BCl₃) and sulfurhexafluoride (SF₆) to form a number of metal contact openings. Thebarrier layer is formed on a channel layer, and includes gallium nitride(GaN). Each of the metal contact openings has a bottom surface that liesabove and spaced apart from a top surface of the channel layer. Themethod also includes etching the barrier layer exposed by the metalcontact openings with a gas combination that includes boron trichloride(BCl₃) and chlorine (Cl₂) to deepen each metal contact opening to asecond bottom surface. The second bottom surface lies above and spacedapart from the top surface of the channel layer.

A better understanding of the features and advantages of the presentinvention will be obtained by reference to the following detaileddescription and accompanying drawings which set forth an illustrativeembodiment in which the principals of the invention are utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-5 are a series of cross-sectional views illustrating an exampleof a method 100 of forming a group III-N HEMT in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-5 show a series of cross-sectional views that illustrate anexample of a method 100 of forming a group III-N HEMT in accordance withthe present invention. As described in greater detail below, the methodof the present invention utilizes a two-step etch process to form metalcontact openings in a group III-N HEMT with depths that are easilycontrolled and do not extend into the channel layer.

As shown in FIG. 1, method 100 utilizes a conventionally-formed groupIII-N HEMT 108. HEMT 108, in turn, includes a single-crystal,lightly-doped, p-type silicon semiconductor substrate 110 (e.g., <111>),and a layered structure 112 that is formed on the top surface ofsubstrate 110.

Layered structure 112, in turn, includes a buffer layer 114 that touchessubstrate 110, a channel layer 116 that touches buffer layer 114, and abarrier layer 118 that touches channel layer 116. Further, layeredstructure 112 can optionally include a cap layer 120 that lies overbarrier layer 118.

Buffer layer 114 provides a transition layer between substrate 100 andchannel layer 116 as a result of lattice mismatches. Channel layer 116and barrier layer 118 have different polarization properties and bandgaps that induce the formation of a two-dimensional electron gas (2DEG)that lies at the top of channel layer 116. Cap layer 120 providesenhanced reliability.

Each of the layers in layered structure 112 can be implemented with oneor more sequential group-III nitride layers, with the group-IIIincluding one or more of In, Ga, and Al. For example, buffer layer 114can be implemented with sequential layers of AlN (a thermally-stablematerial), AlGaN, and GaN. In addition, channel layer 116 can be formedfrom GaN, while barrier layer 118 can be formed from AlGaN. Further, caplayer 120 can be formed from GaN.

Further, HEMT 108 includes a passivation layer 122 that touches the topsurface of layered structure 112 (on the top surface of cap layer 120 ifpresent, or the top surface of barrier layer 118 when cap layer 120 isnot present). Passivation layer 122 can be implemented with, forexample, a silicon nitride layer.

As further shown in FIG. 1, method 100 begins by forming a patternedphotoresist layer 124 on passivation layer 122. Patterned photoresistlayer 124 is formed in conventional manner, which includes depositing alayer of photoresist, projecting a light through a patterned black/clearglass plate known as a mask to form a patterned image on the layer ofphotoresist to soften the photoresist regions exposed by the light, andremoving the softened photoresist regions.

As shown in FIG. 2, after patterned photoresist layer 124 has beenformed, the exposed regions of passivation layer 122, the underlyingportions of cap layer 120 (when present), and the underlying portions ofbarrier layer 118 are dry etched using a gas combination that includesboron trichloride (BCl₃) and sulfur hexafluoride (SF₆) to form sourceand drain metal contact openings 132.

Each metal contact opening 132 has a bottom surface 136 that lies aboveand spaced apart from the top surface of channel layer 116. In thepresent example, the following etch conditions are used:

Pressure: 19 mT-21 mT (preferably 20 mT);TCP RF: 200W-400W (preferably 300W);Bias RF: 47.5W-52.5W (preferably 50W);BCl₃: 20 ccm-30 ccm (preferably 25 ccm);SF₆: 45 ccm-65 ccm (preferably 55 ccm);He Clamp: 5T-10T (preferably 6T); andTemp: 45 degC-65 degC (preferably 55C).

The BCl₃ and SF₆ gas combination under the above conditions etches downinto barrier layer 118 for a period of time, but then etchessubstantially no deeper into barrier layer 118 after the period of time.For example, the BCl₃ and SF₆ gas combination under the above preferredconditions etches down into an AlGaN barrier layer 118 a distance ofapproximately 43 Å during an etch time of 65 seconds.

However, from 65 seconds to 200 seconds, the BCl₃ and SF₆ gascombination etches substantially no deeper into the AlGaN barrier layer118. Thus, barrier layer 118 is etched with the BCl₃ and SF₆ gascombination for a predefined time that is equal to or greater than theperiod of time.

As shown in FIG. 3, after the BCl₃ and SF₆ etch, the gas is changed andthe regions of barrier layer 118 exposed by the metal contact openings132 are dry etched for a predetermined period of time using a gascombination that includes BCl₃ and CL₂ to deepen each bottom surface 136to a lower bottom surface 140. In the present example, the BCl₃ and CL₂gas combination etches more of barrier layer 118 than does the BCl₃ andSF₆ gas combination.

Each lower bottom surface 140 lies above and spaced apart from the topsurface of channel layer 116 by a separation distance D. After the etch,patterned photoresist layer 124 is removed in a conventional manner,such as with an ash process. In the present example, the following etchconditions are used:

Pressure: 14 mT-16 mT (preferably 15 mT);TCP RF: 200W-400W (preferably 300W);Bias RF: 8W-12W (preferably 10W);BCl₃: 70 ccm-90 ccm (preferably 80 ccm);Cl₂: 10 ccm-30 ccm (preferably 20 ccm);He Clamp: 5T-10T (preferably 6T); andTemp: 45 degC-65 degC (preferably 55C).

The BCl₃ and CL₂ gas combination under the above conditions furtheretches down into barrier layer 118 at a (slow) rate of approximately1.05 Å/s. Since the initial depths of the metal contact openings 132 inbarrier layer 118 are each approximately 43 Å, and since the BCl₃ andCL₂ gas etches down into barrier layer 118 at a rate of approximately1.05 Å/s, the final depths of the metal contact openings 132 can beeasily controlled.

For example, if barrier layer 118 is 180 Å thick and 43 Å of barrierlayer 118 have been removed by the BCl₃ and SF₆ etch, then the BCl₃ andCL₂ etch requires approximately 101.9 seconds at a rate of approximately1.05 Å/s to extend each metal contact opening 132 down another 107 Åinto barrier layer 118, thereby forming the lower bottom surfaces 140 tobe 150 Å deep in barrier layer 118 and leaving a 30 Å separationdistance D.

An approximate etch time of 101.9 seconds is substantially longer thanthe few etch seconds available in the prior art, thereby allowing easycontrol of the depths of the metal contact openings 132. As a result,one of the advantages of the present invention is that the depths of thesource and drain metal contact openings 132 can be easily controlled andprevented from exposing or extending into channel layer 116.

As shown in FIG. 4, after the source and drain metal contact openings132 have been deepened to the lower bottom surfaces 140, a metal layer144 is deposited to touch the top surface of passivation layer 122 andfill up the metal contact openings 132 in barrier layer 118, cap layer120, and passivation layer 122. Metal layer 144 is free of gold, and caninclude, for example, a titanium layer, an aluminum copper layer(0.5%Cu) that touches and lies over the titanium layer, and a titaniumnitride cap that touches and lies over the aluminum copper layer.

As shown in FIG. 5, after metal layer 144 has been formed, metal layer144 is planarized in a conventional manner, such as withchemical-mechanical polishing, to expose the top surface of passivationlayer 122. The planarization forms source and drain metal contacts 150in the source and drain metal contact openings 132, respectively. Theplanarization also forms a group III-N HEMT structure 152. The metalcontacts 150, which are free of gold, make ohmic connections to barrierlayer 118. Method 100 then continues with conventional steps to completethe formation of a packaged group III-N HEMT.

It should be understood that the above descriptions are examples of thepresent invention, and that various alternatives of the inventiondescribed herein may be employed in practicing the invention. Forexample, group III-N HEMTs are conventionally formed as depletion-modedevices, but can also be formed as enhancement-mode devices.

The present invention applies equally well to enhancement-mode devicesas the substrate and buffer layer structures of these devices are thesame. Therefore, it is intended that the following claims define thescope of the invention and that structures and methods within the scopeof these claims and their equivalents be covered thereby.

What is claimed is: 1: A method of forming a high electron mobilitytransistor comprising: forming a layered structure including a barrierlayer directly on a channel layer directly on a buffer layer directly ona substrate; forming metal contact openings in the layered structure byetching with a first gas combination, each of the metal contact openingshaving a first bottom surface that lies above and spaced apart from thechannel layer; and etching the layered structure with a second gascombination to deepen each metal contact opening a distance to a secondbottom surface, the second bottom surface lying above and spaced apartfrom the channel layer, wherein the second gas combination etches moreof the barrier layer than does the first gas combination and wherein thefirst gas combination also etches through a cap layer on the barrierlayer, and through a passivation layer on the cap layer, the cap layerincluding GaN, the passivation layer including silicon nitride. 2: Themethod of claim 1, wherein the first gas combination includes borontrichloride (BCl3) and sulfur hexafluoride (SF6), and the second gascombination includes boron trichloride (BCl₃) and chlorine (Cl₂). 3: Themethod of claim 1, further comprising depositing a metal contact layerthat contacts each second bottom surface and fills the metal contactopenings. 4: The method of claim 3 and further comprising planarizingthe metal contact layer to form a number of spaced-apart metal contactsthat lie in the metal contact openings. 5: The method of claim 1,wherein the channel layer comprises GaN. 6: The method of claim 1,wherein the barrier layer is AlGaN. 7: A method of forming a highelectron mobility transistor comprising: forming a channel layer over asubstrate; forming a barrier layer over the channel layer, the barrierlayer including a GaN material; etching the GaN material of the barrierlayer with a gas combination that includes boron trichloride (BCl₃) andsulfur hexafluoride (SF₆) to form a number of metal contact openings,each of the metal contact openings having a bottom surface that liesabove and spaced apart from a top surface of the channel layer; andetching the GaN material of the barrier layer exposed by the metalcontact openings with a gas combination that includes boron trichloride(BCl₃) and chlorine (Cl₂) to deepen each metal contact opening to asecond bottom surface, the second bottom surface lying above and spacedapart from the top surface of the channel layer. 8: The method of claim7, further comprising depositing a metal contact layer that contactseach second bottom surface and fills the metal contact openings. 9: Themethod of claim 8, further comprising planarizing the metal contactlayer to form a number of spaced-apart metal contacts that lie in thenumber of metal contact openings. 10: The method of claim 7, wherein thegas combination that includes BCl₃ and SF₆ also etches through a caplayer over the barrier layer, and through a passivation layer over thecap layer. 11: The method of claim 10, wherein the cap layer includesGaN and the passivation layer includes silicon nitride. 12: The methodof claim 7, wherein the gas combination that includes BCl₃ and SF₆ alsoetches through a passivation layer over the barrier layer. 13: Themethod of claim 12, wherein the passivation layer includes siliconnitride. 14: The method of claim 7, wherein the channel layer includesGaN. 15: The method of claim 7, wherein the GaN material of the barrierlayer is AlGaN. 16: A method of forming a high electron mobilitytransistor comprising: forming a buffer layer over a substrate; forminga channel layer over the buffer layer; forming an AlGaN layer over thechannel layer; forming a cap layer over the AlGaN layer; forming a layerof SiN over the cap layer; forming a source contact and a drain contactthrough the layer of SiN, the cap layer, and into the layer of AlGaN,wherein a bottom surface of the source contact and a bottom surface ofthe drain contact are within the AlGaN layer and are spaced apart fromthe channel layer. 17: The method of claim 16, wherein the channel layercomprises GaN. 18: The method of claim 16, wherein the cap layercomprises GaN. 19: The method of claim 16, wherein forming the sourcecontact and the drain contact comprises: etching the layer of SiN, caplayer, and AlGaN layer with a gas combination that includes borontrichloride (BCl₃) and sulfur hexafluoride (SF₆) to form a sourcecontact opening and a drain contact opening; etching the AlGaN layerwith a second gas combination that includes boron trichloride (BCl₃) andchlorine (Cl₂) to deepen the source contact opening and the draincontact opening; depositing a metal contact layer in the source contactopening and the drain contact opening; and planarizing the metal contactlayer to form the source contact and the drain contact. 20: The methodof claim 19, wherein the metal contact layer comprises a titanium layer,an aluminum copper layer over the titanium layer, and a titanium nitridelayer over the aluminum copper layer.